router startup behavior

Borchers, Mark mborchers at
Wed Jan 16 14:48:51 UTC 2002

> > Just guessing - you're seeing these events between midnight 
> > and 5 am?
> Hm, couldn't reist this one: "which time zone"?
> Just hinting that even though it's that time interval in the US, local
> time is different in other places around the world, so if this is
> causing disturbance, others are probably being hit in their working
> hours.

Heh, the "perpetual global maintenance window syndrome", eh?  A
very useful concept...
> Besides, I was under the impression that to activate a new outbound
> roting policy on a Cisco, you could just modify / replace it, but that
> you would still have to do
>   router#clear ip bgp xxx soft out
> to activate it.  This means that the policy for an existing peer can
> be modified without having to remove the peering and reenable it
> shortly thereafter (something which would cause needless route
> flapping).

Somewhat true.  The new policy would not be applied to routes that 
were already in the table, but would be applied to any adds/withdrawals
that occur once the policy changes are placed in the configuration.
This fact has a synergistic effect when you're making changes that
affect lots of sessions.  So a policy change made a significant time
before a clear or soft clear could in fact result in flaps.

Also consider:  there are a lot of routers using traditional ACL's in their 
policies (as opposed to things like prefix-lists which have more granular
editing features) which would necessitate removing the ACL completely and 
rewriting it with updated lines.  Due to the above, a potential for leaks 
exists unless the session is either shutdown or deleted while acl's are 
being modified. 

If the sequence of events in a configuration script is not well thought
out, the result could be what Ratul has observed in his study. 
> Regards,
> - Håvard

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